It is known that poly(ethylene oxide) (hereinafter referred to as PEO) chains are highly soluble in water, are highly flexible, have extremely high motility in water, and form a hydrogel having high biocompatibility. Accordingly, in order, for example, to enhance biocompatibility, it has been proposed to graft a PEO chain to a polymers prepared from other monomers or to form a great variety of block copolymers having a PEO chain as one block.
An example of the former technique of grafting a PEO chain is found in WO 93/16687. It is claimed therein that the resulting polymers can be used for the microencapsulation of various drugs and cells.
As examples of the latter technique, block copolymers having a PEO chain as a hydrophilic domain and a poly(lactide) chain as a hydrophobic domain have been provided by some of the present inventors (see WO 96/32434, WO 96/33233 and WO 97/06202). Since these hydrophilic-hydrophobic block copolymers form a stable polymeric micelle in an aqueous medium, not only they can be used for biocompatible coating purposes, but also attention is being paid to their utilization, for example, as targeting carriers for drugs.
Meanwhile, it is proposed in recent years to use not only various modified viral vectors but also liposomes as means for internalizing genes into animal cells. It is suggested that, as typical examples of such liposomes, ones formed from cationic lipids can serve as excellent carriers for DNA.